1. Field of the Invention
This invention relates to hot melt adhesive compositions which include a neutralized sulfonated polystyrene resin wherein the neutralized sulfonated polystyrene polymer has about 5 to about 250 meq. of neutralized sulfonate groups per 100 grams of the neutralized sulfonated polystyrene polymer, and about 25 to about 200 parts by weight of a hydrocarbon resin of a petroleum or coal tar distillate, having about 5 to about 6 carbon atoms the hydrocarbon resin being composed of aliphatic dienes and mono-olefins per 100 parts by weight of the neutralized sulfonated polystyrene resin.
Broadly speaking, synthetic adhesives used in packaging can be classified into four categories: water based, solvent based, reactive and hot melt adhesives. Of these four, currently the water based are used most extensively. Usually the water based adhesives are based on emulsion polymers and are applied to porous cellulosic substrates. Energy from the outside in some fashion is applied to the system to evaporate the water in order that a strong bond may be formed. Besides this energy requirement for the formation of the bond, there is another complication with the use of water based adhesive. For a uniform coating a good uniform wetting of the substrate surface is desired, which is not easily achieved.
With the solvent-based adhesives usually a good wetting is achieved; however, their use has been becoming extremely restrictive due to expensive energy requirements for the evaporation of organic solvents fire hazards associated with the use of these organic solvents and emissions problems with said solvents. The strict government environmental regulations and restrictions concerning worker's exposure to solvent vapors, etc. has placed extra pressure on the packager to use non-solvent based adhesives.
Recently the use of hot melt adhesives has been growing very rapidly in the packaging industry. The hot melt adhesives are generally applied (as the name implies) by conventional extrusion or coating techniques in the temperature range of 250.degree. to 450.degree. F. on one of the surfaces to be bonded. The other surface is brought in contact with the hot surface for a sufficient period of time for the melt to cool, whereupon solidification a strong and durable bond is formed.
The key requirements of resins suitable for hot melt adhesive applications are that they should have good physical properties, e.g., tensile strength, etc., at ambient conditions, and they can flow easily at fabrication temperatures. The ethylene vinyl acetate (EVA) copolymers and styrene block copolymers, such as Kraton, have been extensively used as hot melt adhesives; however, their use has been limited largely to pressure sensitive adhesives.
There is a significant demand in the marketplace today for polymer systems which exhibit good green strength or tensile properties at ambient temperatures which, when heated to a predetermined temperature, will give good flow such that they may be applied to a coating or substrate by melt extrusion or other suitable techniques. In the past it has been common to employ organic solutions as a way of applying a coating of a polymer system which is designed to have good adhesive properties. This practice is now under considerable pressure, due to the fact that the organic solvents must be evaporated from the coating giving rise to pollution problems and excess energy requirements.
2. Prior Art
Several U.S. Patents have described sulfonated polymers such as sulfonated butyl and sulfonated EPDM in adhesive applications (e.g., U.S. Pat. No. 3,867,247 and U.S. Pat. No. 3,801,531). It is important to distinguish the instant invention over those prior art systems. The former patent is directed at a sulfonated butyl cement which is solvent based and is employed to laminate various substrates. It is important to note that the instant invention differs dramatically from that patent as follows:
(a) The adhesives of the instant invention are not deposited from solvents but are hot melt and require no solvents.
(b) The adhesive of the instant invention incorporates substantial levels of a highly unsaturated hydrocarbon rubber which is a critical component of these systems.
(c) The instant invention may optionally include a preferential plasticizer capable of associating with the metal sulfonate groups and thereby reducing the melt viscosity of the resulting blends to make the systems more processable;
(d) The instant invention is directed at sulfonated polystyrene resin whereas most of the prior art deals with sulfonated Butyl rubber (e.g., U.S. Pat. No. 3,867,247).
With regard to the latter point, historically polystyrene systems do not possess any tack properties at all. This problem becomes even more acute when polystyrene is sulfonated to levels of 5 to 250 milliequivalents (meq.) per 100 grams of polymer and neutralized. The resulting compositions have been widely used as the basis for ionically crosslinked thermoplastics elastomers. The use of these materials in such applications is, in part, a demonstration that the properties of such materials are just the opposite of those desired for adhesive. In fact such materials are remarkably devoid of tack or adhesion. The tack, therefore, of converting such physically crosslinked materials into suitable adhesive candidates is a particularly challenging one for two reasons: (a) the polystyrene backbone is particularly unsuited for that purpose being a very dry thermoplastic; (b) the strong associations attributable to metal sulfonate crosslinks further inhibit adhesion to any particular substrate.
Despite these handicaps there are some very good reasons for solving the problems associated with converting sulfonated polystyrene into a good hot melt adhesive composition. The excellent thermal stability inherent in the polystyrene backbone is a very desirable property for adhesives which will be exposed to high temperatures for long times. Most adhesives based on other elastomeric backbones can suffer degradation under those conditions.